Barack Obama has scuppered Royal Bank of Scotland's plan to raise $4bn (£2.6bn) from the sale of its energy trading arm, Sempra, by discouraging the main bidder for the business from buying the entire operation for fear of the US crackdown on risky trading activities.

The Edinburgh-based bank, in which the taxpayer has a stake of more than 80%, has sold the European and Asian operations to US bank JP Morgan and is continuing to seek a buyer for the US arm, which would be hit by the Volcker rule announced by the US president to stop US banks engaging in proprietary trading.

JP Morgan is paying $1.7bn for the non-US businesses of Sempra, a joint venture with the American company Sempra Energy, which is receiving $940m of the sale price.

As talks over the sale of the US arm continue, RBS is entering detailed discussions with UK Financial Investments, the body that controls the taxpayers' stakes in the bailed-out banks, about the size of its bonus pool for 2009.

With the furore over bankers' bonuses escalating following the £2.2bn payout for investment bankers at Barclays, RBS is hoping to persuade UKFI that its planned bonus payout of £1.3bn is more conservative than Barclays.

RBS is arguing that its plans – already massaged down by UKFI from a proposed pot of £1.6bn – represent less than 30% of the revenues generated by its investment bankers compared with the 38% of ­revenues paid out by Barclays today. UKFI can veto the overall size of the pay pot and Alistair Darling is expected to decide shortly whether the £1.3bn currently being discussed is politically acceptable. The bank reports its 2009 figures next week, by which time the chancellor will have to decide that the figures represent fair value for the taxpayer.

The government has already stopped cash bonuses being paid to RBS bankers earning more than £39,000 and will defer bonuses in three instalments over three years. Last year it paid its bonuses in the bank's debt to avoid paying in cash and while the hope is that shares can be used instead of debt this year, it is thought that a conclusion has still not been reached on the structure of the payments.

Selling the European and Asian Sempra arm might ease some of the dilemma RBS is facing over bonuses as some of the highest paid traders at the bank are believed to work in the commodity trading arm.

Bruce Van Saun, the RBS finance director, defended the sale: "We are pleased to have expeditiously reached agreement on the divestment of these unique assets. We believe we have struck a fair price and contract with JP Morgan."

He said the remaining business was of "high value" and "performing well" and the bank was considering alternative options for the US operations.

When the business was put on the block last year to appease the EU, the original bids came from JP Morgan – which became the favoured bidder – Deutsche Bank of Germany and Australia's Macquarie and were for more than $4bn. There are hopes that the other bidders might be interested in the US operations that are proving difficult for JP Morgan to buy. There is also speculation that Sempra might agree to buy out RBS from the venture.

RBS expects to report a "small gain for this transaction" which involves the sale of $17bn of assets on which a profit was made in the 11 months to 30 November 2009 of $300m. This will be distributed in line with agreed profit-sharing arrangements between the two joint venture partners, the bank said.

"Net cash proceeds to RBS from the sale will be used to repay short-term borrowings, while the net impact on the RBS core tier one capital ratio on completion is expected to be slightly positive," RBS said.